I understand the default styleshee is accessbile at resource://gre-resources/html.css as you say. But my question, apart from the Firefox/Options dialog, are the user override styles saved as a .css file and if so, can they also be accessed. Or, are the local overrides appended to the html.css file in some way.
Does this help?
If a Firefox user wants to override styles on a permanent basis they would edit userContent-example.css and copy it as userContent.css . This file is located in your Firefox profile. It is usually located in the "application data" folder on Windows, or the "application support" folder on Macintosh. For example I'm using Windows 7 on my computer the css file is located at: (windows profile user name)/AppData (which is a hidden folder)/Roaming/Mozilla/Firefox/Profiles/nx3oq5bc.default/chrome/userContent-example.css
If a user does override their styles on a permanent basis then as a developer you would have no control over the finished product (how it would look to the user).
If as a developer you want to give the user some choices (different stylesheets to choose from) try something like this:
<link href="default.css" rel="stylesheet" type="text/css" title="Default Style">
<link href="fancy.css" rel="alternate stylesheet" type="text/css" title="Fancy">
<link href="basic.css" rel="alternate stylesheet" type="text/css" title="Basic">
With these three style sheets offered, the styles "Default Style", "Fancy", and "Basic" will be listed in the View >Page Style > submenu. When the user selects a style, the page will immediately be re-rendered using that style sheet.
When style sheets are referenced with a title attribute on the <link> or <style> element, the title becomes one of the choices offered to the user. Style sheets linked with the same title are part of the same choice. Style sheets linked without a title attribute are always applied.
Use rel="stylesheet" to link to the default style, and rel="alternate stylesheet" to link to alternative style sheets. This tells Firefox which style sheet title should be selected by default, and makes that default selection apply in browsers that do not support alternate style sheets.
Hope that helps answer your question?